It is known to heat the drum of a laundry dryer directly by means of an electrical resistance heating element. DE 43 13 538 A1, for example, describes a device for drying textile material in dryer drums. In order to deliver heat from the heater as directly as possible to the textile material to be dried, one or more electrical resistance heating elements are disposed on the outer or inner surface of the cylindrical drum wall and fixedly attached thereto. This aims at increasing the efficiency of the laundry drying process.
The disadvantage here is that an electrical connection must be provided to the electrical resistance heating elements, which rotate with the drum. This can be achieved using, for example, brush contacts. However, this is complex and therefore expensive. Moreover, the brush contacts are subject to considerable wear, so that such approaches for electrically contacting relatively moving contact partners have a limited service life. This may result in increased maintenance and servicing costs of such laundry dryers.
Another disadvantage is that the generation of heat is accomplished directly by electric current, which may result in high power consumption of such electrically heated laundry dryers. This leads to an inefficient way of drying laundry, which, today, is undesirable in view of increasing electricity costs and from an environmental impact point of view.
Also known in the art are heat pump dryers. These have a closed heat pump circuit, including a compressor, an evaporator, a condenser and a restriction device (e.g., a capillary tube or an expansion valve). Via this heat pump circuit, moisture that has previously been removed from the laundry is removed from the process air. To this end, the process air previously heated and dehumidified by the heat pump circuit is delivered through an air supply duct into a drum of the laundry dryer by means of a fan. In the drum, the laundry to be dried is typically moved by rotation so that the process air can reach the laundry as completely and uniformly as possible.
In the process, the heated process air absorbs moisture from the laundry, thereby drying it. The moist process air is then returned via an air return duct to the heat pump circuit. There, the moisture removed from the laundry is condensed from the process air and discharged in liquid form to the outside. The energy extracted from the air in this process is returned to the process air, so that the process air exits the heat pump circuit in a reheated condition in a direction toward the drum. The process air cycle is thereby closed. Examples of heat pump dryers are found in EP 2 642 018 A2 and DE 42 12 700 A1.
A heat pump dryer is a condenser dryer that heats the process air by convection. The process air heats the laundry by convection and evaporates the water. Subsequently, the warm and moist air is dehumidified and cooled in the air condenser (heat pump evaporator). In this connection, it is necessary to ensure superheating prior to entry into the compressor of the implemented heat pump circuit; i.e., the compressor may only draw in dry steam, but no two-phase mixture, because this would result in failure of the compressor.
The state of the refrigerant is highly dependent on pressure and temperature. These two variables, and thus the entire heat transfer process, are strongly influenced by the enthalpy flow of the process air and the incipient condensation at the heat transfer surface of the air condenser. This means that, in order to minimize the drying time, the process must be controlled so as to increase the enthalpy flow of the process air, to adjust it to the operating range of the heat pump, to ensure superheating, and at the same time to condense as much water as possible from the process air.
While heat pump dryers are significantly more energy-efficient than laundry dryers having electrical resistance heating elements, the temperature range they can achieve with their heat pump is significantly smaller and at a lower temperature level. This can lead to significantly longer drying times, which may result in increased stress on the laundry due to the increased duration of the mechanical movement. Also, especially at the beginning of the drying process, it can take a relatively long time for the drum to heat to the target temperature. This also increases the drying time.
In order to assist and speed up the drying process, and thereby also minimize the stress on the laundry, an additional heating source may be provided. It is generally known to heat the drum of a laundry dryer by means of an induction heater. For optimum process operation and minimum drying time, the drum temperature must be measured and controlled by adjusting the heat output. This requires that the drum temperature be measured with sufficient accuracy.
One way of doing this is to measure the temperature of the drum directly using an external sensor, such as an infrared sensor. However, this is disadvantageous for various reasons. On the one hand, this adds to the complexity and expense of manufacturing the respective appliance, especially if, for example, a black coating has to be applied to the outside the drum to ensure proper temperature measurement. On the other hand, this increases the risk of failure; i.e., reduces the reliability of the appliance, because the sensor may fail or become unable to measure properly. Optical infrared sensors, for example, may easily become contaminated, for example, by lint, which inevitably forms in the dryer. A possibly required outer coating of the drum may change its properties with time or become damaged, and thus also impair the reliability of the measurement.